This invention relates to packaging of optoelectronic components which generate or process signals that pass through optical fibers. In particular, it addresses the critical need for providing stable, low-cost alignment of single-mode optical fibers to a single packaged device, such as a semiconductor laser.
An optoelectronic package is a container or housing that provides protection and support for both active and passive components contained within it. These components and their interconnection represent an optical-electrical circuit and define the function of the package. The package also includes a means of connecting the internal components with the external environment, usually as electrical feed-through and optical fiber. Our invention is concerned with optically coupling the optical fiber to the components within the package.
To make an optical connection between an optical fiber and an optoelectronic component within a package, it is necessary to position or align the optical fiber in a way that allows efficient coupling between the optical fiber and the optoelectronic component. The precision needed for the alignment depends on the size of the light-emitting or light receiving elements, the type of optical fiber, and any type of focusing or defocusing elements which may be present. Optical fiber transmits light through its inner core, which is much smaller than the diameter of the optical fiber. There are two classes of optical fiber presently used in packaging semiconductor devices: single-mode and multi-mode, with typical core diameters of about 10 microns and 50 microns, respectively. Many telecommunication applications use single-mode optical fiber because of the superior bandwidth arising from its reduction of mode partition noise.
The prior art for packaging semiconductor lasers is predominantly concerned with the easy task of aligning large cored multi-mode optical fiber. Multi-mode optical fiber is of little value for telecommunications because it suffers from mode-partition noise when used for high speed transmissions over a distance.
Laser packaging with single-mode optical fiber has been done with optical fibers which have their ends either cleaved or tapered and lensed. A cleaved optical fiber has an optically flat end, while a tapered and lensed optical fiber is drawn down to a point in a fashion that aids light entering the fiber. Packages incorporating cleaved optical fibers require a separate lens, as does the alignment method according to the present invention, while packages incorporating lensed optical fibers do not.
Packages utilizing cleaved or tapered and lensed optical fibers suffer from stability problems associated with lateral movement of the optical fiber with respect to the laser. For this reason, the alignment of the optical fiber with the laser for such packages is usually done with expensive piezo-crystal micromanipulators having submicron sensitivity. The optical fiber is fastened with expensive laser welding techniques or special solders.
As explained by Rideout, et al, "Improved LED and laser packaging using up-tapered single mode fibers," CLEO '89, Baltimore, Md., Apr. 25, 1989, the improved lateral tolerances arise from first magnifying the emitting spot image of the laser. The larger spot is then projected congruently onto the corresponding uptapered optical fiber. The lateral and angular sensitivities are: EQU Lateral coupling=exp (-x.sup.2 /W.sub.o.sup.2) EQU Angular decoupling=exp (-.pi..sup.2 W.sub.o.sup.2 .theta..sup.2 /.lambda..sup.2)
where the symbols mean:
W.sub.o =spot radius of the optical fiber;
.lambda.=wavelength
.theta.=angular misalignment
x=lateral misalignment
The equations show that the spot radius of the light beam in the optical fiber determines these sensitivities. Since the spot radius is in the denominator of the lateral decoupling expression above, the benefit of decreased lateral sensitivity occurs with increased spot size. Conversely, the angular sensitivity becomes more detrimental since the spot radius is in the numerator of the angular misalignment expression above.
When performing optical fiber alignments, the lateral alignment is more difficult to achieve than the angular alignment. Thus, the net effect of using a lens to magnify the spot radius of the light beam for coupling it to a larger diameter uptapered optical fiber is beneficial.
It is worth noting that even though the thick section of the uptapered optical fiber does not qualify as a single-mode optical fiber diameter, it is short enough in length that it maintains only the single-mode. Thus, it is possible to obtain the advantage of the ease of alignment of a thick multi-mode optical fiber, while not losing the data transmission advantage of a thin single-mode optical fiber.